Langmuir 2000, 16, 2343-2350
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Total Internal Reflection Sum-Frequency Spectroscopy: A Strategy for Studying Molecular Adsorption on Metal Surfaces Christopher T. Williams,† Yong Yang,†,‡ and Colin D. Bain*,† Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, U.K., and State Key Laboratory for Physical Chemistry of Solid Surfaces and Institute for Physical Chemistry, Xiamen University, Xiamen, Fujian 361005, P.R. China Received July 27, 1999. In Final Form: November 17, 1999 Total internal reflection sum-frequency spectroscopy (TIR-SFS) is shown to be capable of detecting molecules adsorbed on ultrathin gold films (e10 nm) deposited on a sapphire prism. Octadecanethiol (ODT) and thiocyanate (SCN-) were used as probe molecules in order to assess the usefulness of the approach. For ODT adsorbed on 5-nm Au films, SF signal enhancements of over an order of magnitude were observed with TIR-SFS compared to the standard external reflection geometry. While TIR-SF spectra were obtained for ODT on 5- and 10-nm Au films, no molecular signals were detected for 20-nm Au films. The CtN stretch of SCN- adsorbed on a 5-nm Au film was detected by TIR-SFS in the presence of either water or air. A theoretical model is presented to rationalize the different SF signal levels observed under various conditions. Future prospects of TIR-SFS for studying other oxide-supported metals are discussed, along with possible applications in the fields of heterogeneous catalysis and electrochemistry.
1. Introduction Solid-liquid interfaces occur in many important industrial catalytic applications, including fine chemicals production1 and electrochemical fuel cells.2 Developing a molecular-level understanding of adsorption and reactions on solid surfaces in the presence of liquid solutions constitutes an important goal of research in these areas. Due to the presence of the bulk liquid phase, such interfaces are among the most difficult to examine with traditional surface-science approaches. Only a limited number of techniques are capable of probing surface chemistry at solid-liquid interfaces with molecular resolution. Although scanning probe microscopy3a,b and synchrotron-based X-ray techniques3c have recently made an impact in this area, the most widely used approach over the past two decades has been surface vibrational spectroscopy.4 In particular, infrared reflection-absorption spectroscopy (IRAS) and surface-enhanced Raman spectroscopy (SERS) have been used extensively to examine adsorption from aqueous solutions onto metal electrodes. IRAS requires a thin-layer cell to minimize the optical path length in the solution and potentialdifference strategies in order to discriminate interfacial from bulk signals.5 These experimental considerations have largely excluded studies of nonelectrochemical interfaces or of catalytic systems acting under practical * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Oxford. ‡ Xiamen University. (1) See for example: (a) Topics in Catalysis: Fine Chemicals Catalysis - Part 1, Blackmond, D. G., Leitner, W., Eds.; 1997; Vol. 4, p 175-270. (b) Baiker, A. J. Mol. Catal. A: Chemical 1997, 115, 473. (c) De Bellefon, C.; Fouilloux, P. Catal. Rev. - Sci. Eng. 1994, 36, 459. (2) See for example: (a) Wasmus, S.; Kuver, A. J. Electroanal. Chem. 1999, 461, 14. (b) Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998. (3) (a) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (b) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (c) Abrun˜a, H. D. In Advances in Chemical Physics; Prigogine, I., Rice, S. A. Eds.; John Wiley & Sons: New York, 1990; Vol. LXXVII, p 255. (4) Weaver, M. J.; Zou, S. In: Advances in Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, UK, 1998; Vol. 26, p 219.
mass transport conditions. SERS has been extensively used by electrochemists to examine adsorption on polycrystalline metal electrodes.6 In contrast with IRAS, the use of a thin-layer geometry or potential difference tactics is not essential. SERS does, however, require specially prepared metal surfaces with roughness features on the order of 10-100 nm (for electromagnetic enhancement) or on the atomic scale (for chemical enhancement). Since the first demonstration of sum-frequency vibrational spectroscopy (SFS) from surfaces over a decade ago, the use of SFS in surface science research has increased rapidly.7 SFS is based on the second-order nonlinear optical process of sum-frequency generation (SFG), in which the interaction of two pulsed laser beams at an interface induces emission of light at the sum of the two frequencies.8 A pulsed fixed frequency laser (usually in the visible) and a pulsed tunable infrared laser are used. The SF signal is resonantly enhanced when the IR frequency matches that of an SF-active vibrational mode of molecules at an interface. Scanning of the infrared beam over the desired frequency range produces a vibrational spectrum. SFG is a coherent process, and the angle at which light is emitted is set by conservation of momentum parallel to the surface. Within the electric dipole approximation, only molecules adsorbed at the solid surface are SF-active: there is no (5) Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; Chapter 7. (6) Pettinger, B. In Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; Chapter 6. (7) (a) Bain, C. D. J. Chem. Soc., Faraday Trans. 1995, 91, 1281. (b) Bain, C. D. In Modern Characterization Methods of Surfactant Systems; Binks, B. P., Ed.; Marcel Dekker: New York, 1999; Chapter 9. (c) Tadjeddine, A.; Peremans, A. In: Advances in Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: Chichester, UK, 1998; Vol. 26, p 159. (d) Miranda, P. B.; Shen, Y. R. J. Phys. Chem. B 1999, 103, 3292. (e) Somorjai, G. A.; Rupprechter, G. J. Phys. Chem. B 1999, 103, 1623. (8) (a) Shen, Y. R. The Principles of Non-Linear Optics; Wiley: New York, 1984. (b) Heinz, T. F. In Nonlinear Surface Electromagnetic Phenomena; Ponath, H.-E., Stegeman, G. I., Eds.; Elsevier: Amsterdam, 1991; Chapter 5.
10.1021/la991009l CCC: $19.00 © 2000 American Chemical Society Published on Web 01/14/2000
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direct contribution to the SF signal from any gas or liquid in contact with the surface.8 Absorption of the IR radiation by the bulk phase does, however, reduce the intensity of the SF light emitted from an interface. In addition to being surface selective, SFS is sensitive to molecular orientation at interfaces. In favorable cases, quantitative information on orientation can be derived from spectra acquired with various polarizations of the incident and emitted beams.7 SFS is being applied in a wide variety of applications, including heterogeneous catalysis,7e detergency,7b tribology,9 crystal growth,10 and chromatography.11 Two different strategies have been employed to obtain SF spectra from the solid-liquid interface. The first uses the thin-layer geometry developed for electrochemical IRAS measurements in order to minimize absorption of the incident infrared beam by the bulk solution. This geometry has been used to study molecular adsorption on several electrode surfaces (e.g., Pt,12-16 Ag,16-19 Au17,19) in the presence of a variety of aqueous electrolyte solutions. The effect of various liquids on the structure of selfassembled monolayers20 and the structure of surfactant films at hydrophobic surfaces have also been examined.7a The second strategy employs total internal reflection (TIR) of the pump beams in the solid. Total internal reflection allows both incident laser beams to reach the interface without passing through the liquid phase. The large enhancement of both the visible and infrared interfacial electric fields when they are incident near the critical angle, θc, for TIR can increase the intensity of the SF signal by several orders of magnitude.21,22 TIR-SFS has been applied extensively to the study of structure at liquid-liquid interfaces.23 This approach has also been used to examine hexane/quartz,24a water/quartz,24b and acetonitrile/ZrO225 interfaces. For each case, it is the orientation of molecules in the presence of neat liquids adjacent to the surface that gives rise to the SF signal. (9) Fraenkel, R.; Butterworth, G. E.; Bain, C. D. J. Am. Chem. Soc. 1998, 120, 203. (10) Potterton, E. Ph.D. Thesis, University of Oxford, 1997. (11) Goates, S. R. Unpublished results. (12) Friedrich, K. A.; Daum, W.; Klu¨nker, C.; Knabben, D.; Stimming, U.; Ibach, H. Surf. Sci. 1995, 335, 315. (13) Tadjeddine, A.; Peremans, A. J. Electroanal. Chem. 1996, 409, 115. (14) Tadjeddine, A.; Peremans, A. Surf. Sci. 1996, 368, 377. (15) Schmidt, M. E.; Guyot-Sionnest, P. J. Chem. Phys. 1996, 104, 2438. (16) Tadjeddine, A.; Peremans, A.; Guyot-Sionnest, P. Surf. Sci. 1995, 335, 210. (17) Hines, M. A.; Todd, J. A.; Guyot-Sionnest, P. Langmuir 1995, 11, 493. (18) Bowmaker, G. A.; Leger, J.-M.; Le Rille, A.; Melendres, C. A.; Tadjeddine, A. J. Chem. Soc., Faraday Trans. 1998, 94, 1309. (19) Ong, T. H.; Davies, P. B.; Bain, C. D. J. Phys. Chem. 1993, 97, 12047. (20) (a) Guyot-Sionnest, P.; Superfine, R.; Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1. (b) Ong, T. H.; Davies, P. B.; Bain, C. D. Langmuir 1993, 9, 1836. (c) Miranda, P. B.; Pflumio, V.; Saijo, H.; Shen, Y. R. J. Am. Chem. Soc. 1998, 120, 12092. (21) (a) Born, M.; Wolf, E. Principles of Optics, 6th ed., Cambridge University Press: Cambridge, 1980. (b) Dick, B.; Gierulski, A.; Marowski, G. Appl. Phys. B. 1987, 42, 237. (c) Conby, J. C.; Daschbach, J. L.; Richmond, G. L. J. Phys. Chem. 1994, 98, 9688. (22) In practice, one cannot work precisely at the critical angle. There are several reasons for erring towards higher angles rather than lower angles: (i) Working above the critical angle reduces the problem of laser-induced heating of the bulk liquid since energy is only absorbed within the evanescent wave. (ii) The absence of transmitted pump beams reduces the amount of scattered light. (iii) Fresnel coefficients are highly sensitive to angle at incident angles just below θc. Working above θc reduces the variation in the Fresnel coefficients caused by the dispersion in the refractive index of the solid and liquid as the frequency of the IR laser is scanned. (23) Gragson, D. E.; Richmond, G. L. J. Phys. Chem. 1998, 102, 3847. (24) (a) Sefler, G. A.; Du., Q.; Miranda, P. B.; Shen, Y. R. Chem. Phys. Lett. 1995, 235, 347. (b) Du, Q.; Freysz, E.; Shen, Y. R. Phys. Rev. Lett. 1994, 72, 238.
Williams et al.
The TIR geometry has also been used to examine the adsorption of CTAB on fused silica in the presence of aqueous solutions with various surfactant concentrations.7b TIR-SFS has been limited to adsorption on oxides that are transparent to both infrared and visible light: metal surfaces have only been studied by the thin-layer approach. Given the advantages of TIR-SFS, it is desirable to extend this experimental approach to metal surfaces. One of the inherent problems is that electric fields are strongly attenuated by propagation through metal films. The extent of attenuation as a function of film thickness can be understood in terms of the “skin depth”, which is dependent on the imaginary component of the refractive index of the metal.26 Typical skin depths for transition metals are ca. 10 nm in the visible (532 nm) and ca. 15 nm in the IR (3 µm).26 Thus, total internal reflection spectroscopy can only be used to study ultrathin metal films.27,28 In this paper, we show that TIR-SFG can be used to examine molecular adsorption on ultrathin gold films (
